1. Field of the Invention
This invention relates to transmission apparatus for transmitting an audio signal from an audio device to a radio receiver and to a corresponding method of transmitting an audio signal. This invention also relates more generally to a transmitter device which transmits on multiple channels.
2. Discussion of the Related Art
Portable media players are an increasingly popular way for a user to carry around their collection of media content. The media content can include audio content such as music tracks and podcasts as well as video content. A user often wants to connect their media player to the audio system in a vehicle, so that they can listen to the audio content through the speakers of the vehicle's audio system. Some vehicles have dedicated connectors, such as jack sockets or a proprietary interface, but many vehicles lack this feature.
One known way of connecting a media player to a vehicle's audio system is to use a low-powered Frequency Modulated (FM) transmitter. The transmitter receives an audio input from the media payer and modulates this onto one of the frequency channels of the VHF FM radio frequency band (87.5-108 MHz) used for licensed radio broadcasts. In this way, the audio output of the media player can be received in the same manner as a conventional FM radio broadcast on the vehicle's FM radio receiver. Regulatory bodies in many countries now allow unlicensed use of these low-power transmitter devices. The transmitter device stores a single transmission frequency which can typically be selected by a user from one of the frequencies across the VHF FM band. In use, a user manually selects a free channel which is not used by a broadcast radio station and selects this as the frequency at which the low-power transmitter device will operate. The in-vehicle radio receiver is tuned to the same channel.
One of the problems with using a device of this kind is that, as a user drives across a region, they may find that the channel they had selected for the low-power transmission is used by a high-powered licensed transmission of a radio station. This requires the user to manually retune both the transmitter device and the in-vehicle radio receiver to a new channel. This is inconvenient, and can be dangerous if a user attempts this operation whilst driving.
The Radio Data System (RDS) is widely used by broadcasters operating in the VHF FM band. The latest version of the RDS standard is published by the International Electrotechnical Commission (IEC) as IEC 62106 (1999). RDS adds a sub-carrier to the FM multiplex at 57 kHz which carries digital data. As FM transmissions have a limited range, a national radio station has to broadcast on different frequencies in neighboring regions to avoid interference. One of the uses of the RDS data channel is to carry a list of Alternate Frequencies. This is a list of neighbouring VHF FM transmitting stations which broadcast the same radio station. When the radio station is received weakly on the current channel, the receiver can select one of the channels specified in the Alternate Frequency list and retune to that channel.
WO 2006/106379A1 describes a device for low-power transmission of audio data to an RDS-capable radio receiver. The device performs a scan of available radio frequencies and selects one of the available frequencies for transmission. Audio data is transmitted over that frequency in a conventional manner by frequency modulating the carrier frequency. Details of the other available frequencies that were found during the scan are sent to the receiver using the Alternate Frequency (AF) field of an RDS data channel forming part of the transmitted signal. WO 2006/106379 only transmits on a single frequency channel at any time. Under poor transmission conditions, there can be uncertainty as to when the device will select one of the alternate frequencies, and which one of the alternate frequencies it will choose. Also, under conditions of multi-path fading, a receiver can experience poor reception of the channel selected by the transmitter device at a time when the transmitter device considers the selected channel is acceptable.